The present invention is to directed toward an improved plastic package for an integrated circuit die, and a method of making such a package.
Integrated circuit die are conventionally enclosed in plastic packages that provide protection from hostile environments and enable electrical interconnection between the integrated circuit die and printed circuit boards. The elements of such a package include a metal leadframe, an integrated circuit die, bonding material to attach the integrated circuit die to the leadframe, bond wires which electrically connect pads on the integrated circuit die to individual leads of the leadframe, and a hard plastic encapsulant material which covers the other components and forms the exterior of the package.
The leadframe is the central supporting structure of such a package. A portion of the leadframe is internal to the package, i.e., completely surrounded by the plastic encapsulant. Portions of the leads of the leadframe extend eternally from the package and are used to connect the package externally.
Further background information concerning conventional plastic integrated circuit packages and leadframes is contained in chapter 8 of the book Microelectronics Packaging Handbook (1989), which was edited by R. Tummala and E. Rymaszewski, and is published by Van Nostrand Reinhold, 115 Fifth Avenue, New York, N.Y.
A problem with conventional plastic packages is that their internal leadframes limit reduction of the size of the packages. Practitioners have attempted to reduce the size of packages by eliminating internal leadframes, as is shown in U.S. Pat. No. 4,530,152 to Roche et al and U.S. Pat. No. 5,172,214 to Castro, but these packages have numerous disadvantages. The contacts of the package shown by Roche in the ""152 patent have orthogonal side surfaces. Accordingly, the packages are believed to be unreliable because the contacts could easily be pulled from the encapsulant material. The package shown by Castro in the ""214 patent has leads which extend into the body of the package from a lower external surface of the package to the top of the die. These leads are large, and have complex bends. Including such leads in a package would increase manufacturing costs and limit reductions in the lateral size of the package. By contrast, the contacts of the packages within the present invention are simpler, do not have such bends, and allow for packages of smaller lateral size.
The present invention is to directed toward improved plastic packages for housing an integrated circuit die, and to leadframes and methods for making such packages. The packages of the present invention are easier and less expensive to make than conventional plastic packages, and are more reliable and efficiently-sized than conventional packages.
In one embodiment of an assembly method for a package within the present invention, Step 1 provides a metal leadframe. The leadframe includes a rectangular frame, e.g., a square frame. A substantially planar die pad is within and connected to the frame. A plurality of finger-like rectangular tabs extend from the frame toward the die pad without contacting the die pad. The number and location of the tabs around the frame may vary. The die pad and the tabs have peripheral side surfaces which include a reentrant portion(s) and asperities. The reentrant portions and asperities enhance the connection of the die pad and tabs to the plastic encapsulating material.
Step 2 places and attaches an integrated circuit to a first surface of the die pad.
Step 3 electrically connects a bond wire or an equivalent conductor between each bonding pad of the die and a first surface of one of the tabs.
Step 4 places the leadframe on a flat surface, with the die facing upwards, and applies a viscous encapsulant material onto the upward facing first surface of the leadframe. The encapsulant material is then hardened. The encapsulant material covers the die, the bond wires, a first surface of the tabs, the first surface of the die pad, the side surfaces of the die pad and tabs, and all or part of the frames around the die pad. A lower second surface of the leadframe, including a lower second surface of the die pad and tabs, is not covered with encapsulant.
Step 5 plates the exposed surfaces of the leadframe, including the exposed second surfaces of the die pad and tabs with a metal, such as copper, gold, lead-tin solder, tin, nickel, palladium, or any solderable metal.
Step 6 cuts the encapsulated portions of the leadframe with a saw. In particular, step 6 either obliterates the disposable portions of the leadframe, or severs the disposable portions of the leadframe from other components of the leadframe, such as the die pad and tabs, which are to be included in the package. Step 6 also trims the encapsulant material and thereby forms the peripheral sides of the package.
A feature of the packages built by the above described method is that the die pad and contacts (i.e., the severed tabs of the leadframe) of the package are located at the lower first surface of the package. The first surfaces and side surfaces of the die pad and tabs are internal to the package, i.e., covered with encapsulant material, but the second surfaces of the die pad and tabs are not covered by encapsulant material. The die pad and tabs are isolated from each other by encapsulant material.
In a completed package, only the encapsulant material holds the die pad and contacts to the package. The connection of the encapsulant material to the die pad and contacts is enhanced by the reentrant portion(s) and asperities of the side surfaces of the die pad and contacts. The reentrant portions and asperities of the side surfaces of the die pad and contacts function as encapsulant fasteners or lead locks.
Numerous variations of the leadframe, package, and assembly method described above also are described in this application. In one alternative assembly method, a leadframe is provided which allows a plurality of packages to be constructed simultaneously.
A leadframe for constructing a plurality of packages simultaneously includes, for example, a matrix of interconnected rectangular frames. A die pad is within and connected to each of the interconnected frames. A set of tabs extend from each frame toward the sides of the enclosed die pad without contacting the die pad. A subsequent encapsulation step includes applying an encapsulant material onto the surface of the leadframe to which the dies are attached. This step covers the dies and the side surfaces of the die pads and tabs within a single block of encapsulant material. The encapsulant material is then hardened. A cutting step separates individual packages from each other and from the disposable portions of the leadframe. The cutting step also severs the connection between each of the interconnected frames and the die pad and tabs within each frame.